One technology for after-treatment of engine exhaust utilizes selective catalytic reduction (SCR) to enable certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3). NH3 is introduced into an engine exhaust system upstream of an SCR catalyst by injecting urea into an exhaust pathway. The urea entropically decomposes to NH3 under high temperature conditions. The SCR facilitates the reaction between NH3 and NOx to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventor herein, issues may arise upon injecting urea into the exhaust pathway. In one example, urea may be poorly mixed into the exhaust flow (e.g., a first portion of exhaust flow has a higher concentration of urea than a second portion of exhaust flow) which may lead to poor coating of the SCR and poor reactivity between emissions (e.g., NOx) and the SCR. Additionally, overly mixing and agitating the urea in the exhaust can likewise cause issues, such as increased deposits.
Attempts to address insufficient mixing include introducing a mixing device downstream of a urea injector and upstream of the SCR such that the exhaust flow may be homogenous. Other attempts to address urea mixing include a stationary mixing apparatus. One example approach is shown by Cho et al. in U.S. 2013/0104531. Therein, a static mixer is located in an exhaust passage downstream of an external tube for injecting urea. The exhaust gas flows through the exhaust passage and combines with a urea injection before flowing through the static mixer.
However, the inventors herein have recognized potential issues with such systems. As one example, the static mixer described above presents limited mixing capabilities due to a large outlet located in a center of the mixer. In some examples, exhaust may flow directly through the orifice without mixing with other portions of exhaust gas. The static mixer inside the exhaust passage also presents manufacturing and packaging constraints. Varying exhaust passage geometries demand an alteration in the manufacturing of the static mixer for the mixer to tightly fit within the exhaust passage.
In one example, the issues described above may be addressed by a system for a urea injector injecting urea inside a perforated tube housed by a mixing chamber configured to receive exhaust gas, the tube having an outlet end fluidly coupled with an exhaust passage and an SCR catalyst positioned downstream of the exhaust passage. In this way, exhaust gas flows into the mixing chamber and into the perforated tube before flowing through the outlet to the SCR catalyst.
As one example, the exhaust passage is a downstream exhaust passage physically separated from an upstream exhaust passage. The upstream exhaust passage is physically coupled to an inlet of an upstream side of the mixing chamber and directs exhaust flow into the mixing chamber. Urea is injected into an upper portion of the perforated tube through a port vertically higher than the outlet on a downstream side of the mixing chamber. Gas from the upstream exhaust passage flows into the mixing chamber and only flows into the downstream exhaust passage after flowing through the perforated tube. In this way, gas may flow through the tube simultaneous to a urea injection.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.